Sound recording



E. w. KELLOGG scum) RECORDING Fil ed may 29' 2 March 7 Edward mizg'g' m 1/1 4 ratus in use.

Patented Mar. 7, 1944 UNITED STATES PATENT OFFICE SOUND RECORDING Edward w. Kellogg. Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1942, Serial No. 445,038

7 Claims.

This invention relates to sound recording and more particularly to the recording of sound on film by the variable density method. In soundrecording by the variable density method, there have been three general types of recording appa- The light valve, the glow lamp, and the penumbra recorder. Each of these types of apparatus has been adapted to ground noise reduction in sound recording by so arranging the apparatus that the total intensity of the light reaching the film decreases as" the amplitude of the modulation decreases thereby causing the positive print to be relatively dense at portions of low modulation.

With glow lamp recording, it has heretofore been customary to reduce the total current through the glow lamp as the amplitude of the modulation decreased and while this has been satisfactory over a limited portion of the range of some types of 'glow lamps, it has not been satisfactory over the entire range of any type of glow lamp and is quite impractical over any tion is low, but the alternating current left unaffected, and the variations in exposure. will be the same as if the direct current had not been reduced. On the other hand it is obvious that if the mean exposure of the negative is reduced by partial obstruction of the light, both the modulation and the steady light will be reduced in like proportion. I propose therefore, simultaneously with the partial intercepting of the light,

- to increase the strength of the modulation of of the range of other types of glow lamps whose I characteristics change with changes in temperature. For example there are various lamps employing mercury as the vapor through which the arc discharge takes place. The brightness of such lamps depends not only on the current passing through-the lamp, but also on the vapor pressure, which depends in turn on the temperature. With such alamp satisfactory control of film exposure is possible only by maintaining the average power input constant, or in other words maintaining the direct current through the lamp constant, although modulating the current by superposition of alternating currents corresponding to the waves to be recorded.

In the present invention, a novel method and apparatus are provided for applying ground noise reduction to glow lamp recording while maintaining the average glow lamp current constant thereby avoiding the disadvantages of the prior art apparatus of this type. This is accomplished by using a ground noise reduction shutter as an aperture stop in the recordingoptical system. and by combining this with means for compensating for the effect of the shutter in reducing the magnitude of the modulation when it obstructs part of the light. In variable density systems of photographic sound recording, ground noise reduction as has already been stated is accomplished by reducing the average exposure of the negative during periods when the modulation to be recorded is low. This results in a thin negative and a correspondingly dark print.

the lamp input, by changing the amplification of a. variable gain amplifier, or by other expedient having thesame effect. Thus when they shutter vanes cut ofi half the light the gain of the input amplifier would be doubled, Or if the light is three-fourths obstructed the amplifier gain would be quadrupled. This would leave the absolute magnitude of the modulation the same as though the shutter had not been used.

The simplest method of app ying this correction is to derive from the ground noise reduction amplifier a voltage having a. definite relationship to the current which actuates the shutter, and to use this voltage to vary the bias of one or more variable gain stages in the amplifier which feeds the lamp. I have chosen, however, to illustrate that embodiment of my invention which employs the well known negative feedback" principle. In this case the effect of variable gain is produced y y g the amount of feedback, which as will be seen is the direct result of the action of the shutter. 7

One object of the invention is to provide an improved variable density sound recording system. Another object of the invention is to provide a glow lamp type sound recorder with ground noise reduction. a

Another object of the invention is to provide a glow lamp type of variable density sound recorder with ground noise reduction and in which the average lamp current remains substantially constant.

Another object of my invention is to provide a system of glow lamp recording with ground noise reduction, which will operate satisfactorily with lamps of types whose sensitivity depends on temperature.

Another object of my invention is to provide a system of glow lamp recording wherein the ground-noise reduction can be carried through a large range of average negative exposure without resulting distortion.

Another object of the'invention is to provide a glow lamp type of variable density recorder with both ground noise reduction and inverse feed back.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the ccompanying drawing in which:

The single fi ure drawing is a schematic diagram of my improved apparatus showing the recording optical system in some detail.

The sound input may originate in some appropriate device such, for example, as the-microphone it or any equivalent source such as a phonograph, telephone line, or the like. The electricaliinpulses corresponding to the sound waves are amplified by the amplifier II and transmitted to the amplifier I! which operates the recording glow lamp l3. The glow lamp II as usual varies in light intensity in accordance with the instantaneous total current transmitted to it from the amplifier l2. Light from the lamp i3 is transmitted through the slit I4 and is focused in the form of a fine line upon the film F by an. appropriate optical system'such, for example, as the double lens illustrated at I5, ll imaging the slit at ll upon the film I".

A ground noise reduction amplifier ll of any customary type may be provided and this acut'ates the shutter vanes II and II through the solenoid or equivalent magnetic motor. This shutter mechanism may be constructed as described and claimed in Baker Patent 2,102,776. The shutter vanes are located at an aperture stop of the system, so that they do not impair the uniformity of illumination of the line of light onthe film.

The apparatus is provided with a distortion reducing feedback circuit as follows: A selective reflector 22 is provided between the objective II,

It and the film I". This selective reflector is preferably made as described in the article entitled "A New Dichroic Reflector And Its Application to Photocell Monitoring Systems," by G. L. Dimmick. appearing on pages 36 to 44 of the Journal of 'the Society of Motion Picture Engineers for J anuary, 1942. This reflector is so constructed as described in the said article that the light to which the film is sensitive is transmitted to the film. while the ight to which the film is relatively insensitive and to which an appropriate photocell, such as a caesium photocell, is. sensitive is reflected to the photocell 23. For example, the blue end of the spectrum may be entirely transmitted to the film while the red end of the spectrum or only the extreme portion thereof may be reflected to the photocell 23. As described in Wilhelm Patent 2,194,175 and Balsley Reissue Patent 21,907, the output from the photocell 23 may be fed back in inverse phase relation to the input to the amplifier if. The photocell is appropriately polarized by some suitable source of potential as illustrated, for example, by the battery 25 and the audio frequency output from the photocell is applied to the input of the amplifier It. A portion The operation of the device is as follows.

86 put to amplifier II.

10 change in an attempt to oil'set the eil'ect of the shutter. '15. high pass filter 24 is indicated for this purpose, but it is to be understood that the exclusion ofthe steady or slow component of output of amplifier 28 may be accomplished at some 15 other point in the circuit. Another portion of the output of the amplifier 25 may be transmitted through the low pass filter a to the ground noise reduction amplifier II where it serves to correct any inaccuracies in the relation between the go amount of light obstruction by the shutter vanes and the control voltage developed in the ground noise reduction amplifier. In operation, the lamp current is adjusted to a desirable value, and this is retained. Depending on the H and D characzs teristics of the negative and print films, and on whether the recording is to be of the'type known as "straight line, or the type known as' 'ftoe recording, two operating points are selected, one which will give the desired exposure during 80 periodsor little or no modulation, and the other which will give the desired average exposure during periods of high modulation. The ground noise reduction amplifier is then adjusted so that when no modulationispresent, the lower of these two as exposures is given the film, (the shutter vanes in this case obstructing a large portion of light) and when full modulation is present, the shutter vanes are sufiiciently withdrawn to give the film the higher of the two selected exposures.

a The ground noise reduction amplifier may be constructed, for example, asdescribed and claimed in McDowell Reissue Patent 21,389, although a number of other practical designs for ground noise reduction amplifiers are available.

The

amplifier it supplies the glow lamp I! with current having a fixed average or direct current value, which is modulated in accordance with the audio frequency input to the amplifier I2.

The input to the amplifier I2 is supplied from the input device I I, through the amplifier ll Negative feedback is provided through the mirror 22,

the photocell 2: and the amplifier 2: whose output is applied across resistance 21, across the in- Such feedback if adequate in amount establishes a fixed relationship between the modulation of the light reaching the photocell and the voltage output of amplifier ll, thereby practically eliminating distortion, such as so might be due for example to non linearity in the lamp. and also making the light modulation at the photocell (and also at the film) substantially independent of the position of the shutter blades. At low inputs the ground noise reducas tion amplifier II maintains the shutters lll! in a nearly closed position, thereby decreasing the totalilluminationonthefilmFtotheproper degree. This decrease in illumination tends to decrease the relative amplitude of the modulation impressed on the film by the same amount.

When, the shutters are in the relatively closed position and modulation is applied, the reflector 22 which refiects a portion of the recording light beam to the cell 23 directs light of correspondingly low modulation to the photocell. Thus the voltage fed back through amplifier 26 is reduced, which in effect increases the gain of the recordlng'amplifler system and results in increased modulation of the lamp current, suflicient to compensate for the decrease in effective modulation caused. by the shutters I8 and I9. At higher levels of modulation the shutters are caused by the ground noise reduction amplifier II, to open, thereby increasing both the total illumination on the film F. The larger fraction of the light passing through the shutter would tend to result in higher modulation at the film, but coincidentally the feedback is increased and the modulation is maintained at its proper value as determined by the output of amplifier I i.

It will be apparent from the foregoing that the combination of the photocell feedback circuit with the ground noise reduction shutter permits the glow lamp to operate at a constant average current, provides ground noise reduction and automatlcally compensates for any distortion introduced by the ground noise reduction by the operation of the ground noise reduction shutter or by the action of the amplifier.

I claim as my invention:

1. In a sound recording apparatus, a light source, electrical means for varying the brightness of the light source in accordance with sounds to be recorded, means for varying the average amount of light transmitted from said source to a record medium in accordance with the average amplitude of the sounds to be recorded, and means for maintaining the average power of said light source at a substantially constant value.

2. In a sound recording apparatus, a light source, electrical means for varying the brightness of the light source in accordance with sounds to be recorded, shutter means for varying the average amount of light transmitted from said source to a record medium in accordance with the average amplitude of the sounds to be recorded, and feed-back means the light modulation impressed on said record medium in correspondence with the sounds to be recorded.

3. In a sound recording apparatus, a light source of the gas-discharge type, means for varymg the brightness of the light source in accordance with sounds to be recorded while maintaining its average brightness constant, means for varying the average amount of light transmitted from said source to a record medium in accordance with the average amplitude of the sounds to be recorded, and means for maintaining the average power of said source at a substantially constant value.

4. In combination, a gas discharge lamp, an amplifier varying the brightness of the lamp in accordance with sounds to be recorded, a shutter varying the average amount of light transmitted from said lamp to a record medium in accordance with the average amplitude of the sounds to be recorded, and a feed-back amplifier for maintaining the light modulation impressed on said record medium in correspondence withthe sounds to be recorded.

5. An impulse recording system including a light source, electrical means for varying the brightness of said source in accordance with the impulses to be recorded, means for varying the average amount of light transmitted from said source to a record medium in accordance with the average amplitude of said impulses, and means responsive to a component of said light for reducing distortion resulting from an inherent characteristic of said source.

6. An impulse recording system including a light source, electrical means for varying the brightness of said source in accordance with the impulses to be recorded, means for varying the average amount of light transmitted from said source to a record medium in accordance with the average amplitude of said impulses, a light selective filter interposed in the path of said light,

and means including a negative feed-back channel interposed between said filter and said source for reducing distortion inherent in the operation of said source.

for maintaining '1. An impulse recording system including alight source, electrical means for varying the bright ness of said source in accordancewith the impulses to be recorded, means for varying the average. amount of light transmitted from said source to a record medium in accordance with the average amplitude of said impulses, a light selective filter interposed in the path of said light, and means for reducing distortion inherent in the operation of said average amount of light varying means.

EDWARD w. KELLOGG. 

